FIG. 1 shows an example of one known turbine engine 10 having a compressor section 12, a combustor section 14 and a turbine section 16. In the turbine section 16, there are alternating rows of stationary airfoils 18 (commonly referred to as vanes) and rotating airfoils 20 (commonly referred to as blades). Each row of blades 20 is formed by a plurality of airfoils 20 attached to a disc 22 provided on a rotor 24, which includes a plurality of axially-spaced discs 22. The blades 20 can extend radially outward from the discs 22 and terminate in a region known as the blade tip 26. Each row of vanes 18 is formed by attaching a plurality of airfoils 18 to a blade ring or vane carrier 28. The vanes 18 can extend radially inward from the inner peripheral surface 30 of the vane carrier 28. The vane carrier 28 is attached to an outer casing 32, which encloses the turbine section 16 of the engine 10.
Between the rows of vanes 18, a ring seal 34 can be attached to the inner peripheral surface 30 of the vane carrier 28. The ring seal 34 acts as the hot gas path guide between the rows of vanes 18 at the locations of the rotating blades 20. The ring seal 34 can be formed by a plurality of ring segments. Some turbine engines use metal ring segments that attach directly to the vane carrier 28, such as by bolts or by providing hooks on the ring seal that are received in a mating slot in the vane carrier 28. In other engine designs, the individual ring segments can be indirectly attached to the vane carrier 28. For example, metal isolation rings (not shown) can be attached to and extend radially inward from the vane carrier 28, and the ring segments can be fixed to the isolation rings by, for example, one or more fasteners. Each ring seal 34 can substantially surround a row of blades 20 such that the tips 26 of the rotating blades 20 are in close proximity to the ring seal 34. The space between the blade tips 26 and the ring seal 34 is referred to as the blade tip clearance C.
In operation, hot gases from the combustor section 14 are routed to the turbine section 16. The gas flows through the rows of stationary airfoils 18 and rotating airfoils 20 in the turbine section 16. Gas leakage can occur through the blade tip clearances C, resulting in measurable decreases in engine power and efficiency. Thus, it is preferred if the blade tip clearances C are kept as small as possible to minimize such gas leakage. However, it is critical to maintain a clearance C at all times; rubbing of any of the rotating and stationary components can lead to substantial component damage, performance degradation, and extended downtime.
During transient operating conditions such as engine startup or part load operation, it can be difficult to ensure that adequate blade tip clearances C are maintained because the rotating parts and the stationary parts thermally expand at different rates. As a result, the blade tip clearances C can actually decrease during transient engine operation, raising concerns of blade tip rubbing.
The blade tip clearances C can further be affected by the differing rates of thermal expansion of the stationary components. In particular, the ring seal 34 is a relatively thin component compared to the other stationary components to which it is operatively connected (i.e., the vane carrier 28 and the outer casing 32). Thus, while the ring seal 34 itself may be able to respond relatively rapidly to operational temperature increases, further radial movement and/or expansion of the ring seal 34 can be impeded by the relatively slower thermal response of the vane carrier 28 and/or outer casing 32.
Eventually, the outer casing 32 and the vane carrier 28 become sufficiently heated and expand radially outward. Because the ring seal 34 is connected to the vane carrier 28 and outer casing 32, the radial expansion of the vane carrier 28 and outer casing 32 can move the ring seal 34 radially outward as well so as to increase the clearances C. At steady state engine operation, such as at base load, the clearances C can become overly large, thereby reducing the efficiency of the engine 10.
Management of blade tip clearances during engine operation is a longstanding issue with turbine engines; various systems and methods have been directed to that goal. For instance, an active clearance controller has been used to adjust the radial position of the ring seal 34 based on an on-line measurement of the tip clearance C. However, such active clearance control systems are complicated and expensive.
Thus, there is a need for a system that can facilitate the optimization of the operational turbine tip clearances C.